The aim of this research is to understand the regulation of protein synthesis in polio virus-infected cells. Infected cells show a rapid and marked inhibition of host cell protein synthesis, during which initiation of new polypeptide chains ceases. We have shown that preparations of crude initiation factors isolated from infected cells fail to promote initiation of translation of cellular mRNA, although they appear fully able to support viral translation. Purification and assay of individual initiation factors yielded little or no eIF-3 activity from infected cells, although the protein can be detected antigenically. We propose to analyze this defective protein both structurally and functionally, with particular emphasis on cap-binding activity of eIF-3 and the associated cap-binding protein. In the infected cell, synthesis of viral proteins proceeds under conditions where cellular protein synthesis remains restricted. We therefore undertook studies of the initiation mechanism for translation of viral RNA. Two unique initiation sites were found to be utilized in vitro, resulting in two amino-terminally labelled tryptic peptides. One site produces capsid protein precursor, and we are proposing studies to identify the other independently-initiated protein. We hope to locate the two ribosome-binding sites on the viral RNA, and to determine whether both initiation sites are utilized in vivo.